Used in industrial process measurement technology are so-called field devices. These are on-site measuring devices for producing analog or digital measurement signals representing measured variables of a process. Field devices are used especially in connection with the automation of chemical or other kinds of processing plants. Examples of process measured variables include mass flow rate, fill, or limit, level, pressure and temperature. Field devices of this kind are described, for example, in EP-A 984 248, U.S. Pat. No. 3,878,725, U.S. Pat. No. 4,308,754, U.S. Pat. No. 4,468,971, U.S. Pat. No. 4,574,328, U.S. Pat. No. 4,594,584, U.S. Pat. No. 4,617,607, U.S. Pat. No. 4,716,770, U.S. Pat. No. 4,850,213, U.S. Pat. No. 5,052,230, U.S. Pat. No. 5,131,279, U.S. Pat. No. 5,363,341, U.S. Pat. No. 5,796,011, U.S. Pat. No. 6,236,322, U.S. Pat. No. 6,397,683 or WO-A 00 36 379.
For the registering of one or more measured variables of a process, the measuring device includes a suitable measurement pickup, which is, most often, a physical-electrical transducer, which is inserted into a wall of a relevant container, e.g. a pipeline or a tank, carrying e.g. a liquid, powdered, vaporous or gaseous, process medium. The transducer serves to produce at least one, especially electrical, measurement signal representing the measured variable of the process.
The measurement pickup is also electrically connected with an appropriate measuring and operating electronics—or, shorter, “measuring device electronics”—serving especially also for a further processing or evaluation of the at least one measurement signal. The measuring device electronics is arranged in most of such measuring devices in the immediate vicinity of the measurement pickup. Additionally, field devices of the described kind are mostly also connected with one another and/or with appropriate process control computers via a data-transmission system connected to the measuring device electronics. The measurement signals are transmitted via (4 to 20 mA)-current loops and/or via digital data bus. Serving as data transmission systems in such case are, especially serial, fieldbus systems, such as e.g. PROFIBUS-PA, FOUNDATION FIELDBUS, together with the corresponding transmission protocols. By means of the process control computers, the transmitted measurement signals can be processed further and visualized as corresponding measurement results e.g. on monitors and/or they can be converted into control signals for process actuators, such as e.g. magnetic valves, electromotors, etc.
For accommodating the measuring device electronics, process measuring devices of the described kind include an electronics housing, which, as proposed e.g. in U.S. Pat. No. 6,397,683 or WO-A 00 36 379, can be located remotely from the measuring device and connected with such only via a flexible cable. Usually, however, the measuring device electronics is accommodated in a corresponding electronics housing of the measuring device, which, as shown e.g. also in EP-A 903,651 or EP-A 1 008 836, is mounted directly on the measurement pickup via a connection element, usually in the form of a neck- or nozzle-shaped connection element or also in the form of a simple flange connection. Often, the electronics housing then serves also for accommodating some mechanical components of the measurement pickup, such as e.g. membrane-, rod- or sleeve-shaped deformation- or vibration-bodies deforming during operation under the influence of mechanical loads, as shown e.g. in EP-A 984 248, U.S. Pat. No. 4,594,584, U.S. Pat. No. 4,716,770 or U.S. Pat. No. 6,352,000. For the electrical connection of measuring and operating electronics to the measurement pickup, connection lines are provided, these being run through the connection element and being sectionally embedded by a sealing material, e.g. plastic and/or glass, usually filling the connection element, at least partially.
The advantage of a direct, especially also rigid, mechanical connection between electronics housing and measurement pickup is, above all, that, on-site, after installation of the measurement pickup, practically no further mounting steps are necessary for the attachment of the electronics housing.
Measuring devices of the described kind must, in industry, satisfy high safety standards, which are, as a rule, rigidly set nationally in corresponding safety regulations, in order that, both in normal operation and in the case of an occurring interruption, a high measure of safety is assured for persons and plant. An important aspect of this is to construct the housing such that it can be subjected safely both during normal operation, and during an interruption, to pressure, or gage pressure. Pressure, or pressure difference, measurement pickups must, for example, be able to withstand pressures up to two-times pick-up-specific, nominal pressure. The nominal pressure is an upper limit value for a pressure acting on the measurement pickup, for which the pressure, or pressure-difference, measurement pickup is designed. Components, such as, for example, connection elements, especially flanges, and screws, must even be able to withstand pressures up to three-times nominal pressure. At three-times nominal pressure, small leaks at sealing locations are, however, tolerable. As a rule, the components of the measurement pickups are, however, irreversibly damaged at a loading of three-times nominal pressure. Such irreversible damage occurs e.g. when components, such as e.g. flanges or screws, are strained beyond yield. The affected parts must then be replaced. In order to prevent this, it is usual that plants have safety valves or burst disks, which open in the case of an exceeding of a prescribed pressure and prevent further pressure increase. Basic requirements in this regard are provided e.g. in the Technical Rules for Pressurized Gases (TRG 250 and 254) and are used in the case of so-called unharmful gases, e.g. inert gases or carbon dioxide, as well as in the case of liquids.
Such an excess pressure protection apparatus placed in front of a measurement pickup in a plant is, however, not tuned to the requirements of the particular measurement pickup and often reacts too slowly to dynamic overloads. In the case of rapid pressure rises, irreversible damaging of the measurement pickup can, therefore, still occur, despite the interposed excess pressure protection apparatus. On top of this, defects can arise with regard to the installation of the interposed excess pressure protection apparatus, e.g. the protection apparatus is completely omitted, or, through inadvertence, an excess pressure protection apparatus is installed, which opens already at pressures below the nominal pressure, or does not open at pressures above three-times the nominal pressure.
Also in the case of flow measuring devices, there is, as a rule, the necessity of building the measurement pickup housing such that it can withstand pressure. In such case, the measurement pickup housing is a tubular jacket, in which the measuring tube extends. The medium, whose flow rate is to be registered, flows, during operation, through the measuring tube. In the case of magnetoinductive flow measuring devices, coils are provided at the measuring tube for producing a magnetic field in the interior of the measuring tube, this being accompanied by electrodes for sampling an induced voltage produced by a flow of a conductive medium through the magnetic field. In normal operation, no high pressures are present in the tubular jacket. However, in the case of a malfunction, e.g. in the case of damage to the measuring tube or to the supply and removal lines, pressure increases can occur in the tubular jacket. For, in such case, assuring an appropriate degree of safety, it is necessary to construct the tubular jacket to be pressure resistant. This is, however, associated with increased costs. Insertion of an excess pressure protection apparatus in front, such as is usual in pressure measurement technology, is not, as a rule, feasible, due to the way in which the flow measurement pickup is constructed. The above considerations are analogously true also for other types of flow measuring pickups, such as e.g. vortex, Coriolis, ultrasonic flow measuring devices or thermal flow meters. Also, in the case of a pressure resistant construction of the tubular jacket, especially in the case of malfunctions in which high and/or rapid pressure increases occur, it is possible to experience irreversible damage to components, such as tubular jacket, screws and flanges, which necessitates replacement of such components.
In the case of a pressure overload, it is, however, not only the measurement pickup itself which is in danger. There is also the danger, that the medium causing the pressure overload can escape uncontrollably. Exactly in large plants, where many measuring devices with measurement pickups are installed, there is frequently a much-branched system of cable conduits, via which the separate measuring and operating electronics are, as shown e.g. in WO-A 03/040851, connected to superordinated process control systems situated most often in remote control rooms. In such plants, especially gaseous and/or vaporous media can, in the case of a pressure overload, possibly reach through the connection element and the electronics housing into the widely branched cable conduit system and spread in the plant completely without control. While, in the vicinity of the measurement pickup, safety measures tailored to the media being used prevail, such is not compelled to be the case in remote areas. Such an uncontrolled escape of an, especially gaseous and/or vaporous medium thus represents a high safety risk. In the case of the measuring device disclosed in WO-A 03/040851, an alarm signal is produced in the case of such a malfunction. However, the forwarding of the medium into the control room can neither be prevented in this manner nor even near-term restrained.